1. Field of the Invention
The present invention relates to a process for preparing a water-soluble polymer, more particularly, to a process for preparing a modified or at least hydrolyzed product of a water-soluble N-vinylcarboxylic acid amide (co)polymer.
2. Description of the Prior Art
Water-soluble polymers containing vinylamine units are expected to be utilized as flocculants and paper-treating agents having excellent benefits. Generally, such water-soluble polymers may be prepared by polymerizing an N-vinylcarboxylic acid amide alone or together with other ethylenically unsaturated monomer(s) followed by acidic or basic hydrolysis, as shown in Japanese Patent Publication (Kokoku) No. 63-9523, and Japanese Patent Application Laying-open (Kokai) Nos. 62-74902 and 63-218718.
In such conventional processes, however, the products in the hydrolysis step tend to form gels or be insolubilized in water, making it difficult to produce powdery water-soluble polymers with good quality efficiently.
The present inventors have investigated the cause of the problem and found that gels tend to be formed when a larger amount of unreacted N-vinylcarboxylic acid amide monomer remains. Generally, such unreacted monomers may be reduced by increasing the reaction period or polymerization temperature; however, these methods are not preferable since the polymers are not always stable. Another method may be elimination of monomers by washing after polymerization; however, this leads to a complicated process and is not industrially advantageous.